I've heard alot of good things on the Tamron 17-50 and I'm about to
purchase one... There are several threads on that lense, from what
I've read you get alot of bang for the buck. Since it is 1/3 the
price of the Nikon 17-55 2.8
--
k2bikerider

Second that... the versatility of a fast (e.g., 2.8) mid-zoom is
second to none. And the IQ bests the mega-zooms by a mile. I use
the Sigma 18-50/2.8 Macro and have been very impressed with its
performance.

I've got the Sigma 18-50/2.8 as well as the 18-200VR. I definitely would disagree that the Sigma bests the 18-200VR "by a mile." It is a bit sharper and has less distortion, but the 18-200VR isn't that bad, either.

If your yardstick says the 18-50/2.8 beats the 18-200VR by a mile, then that same yardstick would also indicate that the 17-55/2.8 beats the 18-50/2.8 by that same mile. In other words, yes, you can see differences, but for the majority of prints, you'll be unlikely to notice the difference.

All three of these lenses are quite good and they have to be for the prices they're commanding.

As for versatility, that's a tough call. I used to think my Sigma 18-50/2.8 was my most versatile lens, but now that I've used the 18-200VR for a while, I'm not so sure anymore. The VR makes up for the lack of f/2.8 in a great many of MY shooting situations. I still use my 18-50/2.8 but for the past few mos., the 18-200 has spent more time on my camera. But now that winter is here and daylight hours are at a minimum, my 18-50/2.8 is seeing more time on my camera again. It'd be very tough if I had to choose just one, but if I had to, the 18-200VR would probably win out.